The invention generally relates to mechanisms for positioning airfoils movably mounted on the wings of an aircraft and more particularly, to mechanisms for controlling the sequence of deployment and retraction of leading edge and trailing edge flaps in response to a single, pilot operated flap control lever.
The sequence of deployment and retraction of leading edge flaps and trailing edge flaps is normally coordinated in a predetermined manner to effect certain predictable aerodynamic reactions. These reactions are in turn designed to cause the desired flight performance of the aircraft, and to prevent certain undesired airfoil configurations that result from particular combinations of relative positions of the leading and trailing edge flaps. These aerodynamic design considerations will vary from aircraft to aircraft depending upon such parameters as the body style, number of engines, weight carrying capability, etc. For example, on the current three and four engine Boeing aircraft models, 727's and 747's, respectively, manufactured by The Boeing Company of Seattle, Wash., flap actuation is coordinated so that the leading edge flaps are not extendable until the trailing edge flaps have been extended to or beyond a predetermined degree of deployment. And, during retraction, the trailing edge flaps must be retracted first, at least to a predetermined point, before retraction of the leading edge flaps can commence.
With this particular sequence of flap extension and retraction, a relatively uncomplicated control mechanism can be, and currently is, used to effect the desired positioning of the flaps in response to a single, pilot operated lever. By shifting the control lever to a position commanding flap extension, a cam operated hydraulic subsystem is activated which hydraulically deploys the trailing edge flaps toward the extended position. When the trailing edge flaps reach the above mentioned predetermined point of deployment, another cam operated hydraulic subsystem is activate, which effects deployment of the leading edge flaps toward full extension. The flaps are retracted in a reverse operating sequence in response to movement of the control lever back to a position commanding flap retraction. The trailing edge flaps retract first to the above mentioned point of deployment, at which time the leading edge flaps automatically commence retraction. It is thus seen that the particular extension and retraction sequence of the two sets of flaps lends itself to a relatively simple control system in which the pilot operated lever is in direct control of the trailing edge flaps, and the leading edge flaps are automatically slaved by a hydraulic subsystem to the position of the trailing edge flaps.
However, other models of aircraft, for reasons of airfoil configuration, weight, engine thrust, etc., require a different operating sequence of the leading and trailing edge flaps. In particular, it has been found necessary on certain aircraft to provide for independent deployment of the leading edge flaps, without first deploying the trailing edge flaps. Such prior extension of the leading edge flaps serves to enhance the climb-out capability of the aircraft. By extending the leading edge flaps, without the trailing edge flaps, a greater lift-to-drag ratio is achieved that permits a greater angle of attack, which in turn allows a sharper climb gradient. The result is an improved climb-out performance that is sometimes needed at high altitude airports, to compensate for the lighter air which diminishes the effectiveness of the engine thrust.
Implementation of the leading-edge-only-flap-extension is, however, not easily accomplished. One solution would provide totally separate, independently variable pilot operated control levers, one for each of the leading and trailing edge flaps. This option is, however, not acceptable because of the added pilot workload (a factor which is always minimized to the greatest possible extent) and, most importantly, because of the increased chance of pilot error, under critical flight conditions, when split second coordination of two flap control levers is required.
Another relatively easily implemented solution would be to provide the preferred, single pilot control lever, in which a first portion of the control stroke of the lever serves to position the leading edge flaps only, and the balance of the control stroke serves to actuate the trailing edge flaps. In other words, by maintaining the pilot control lever in the first portion of the control stroke, the pilot would have independent control over the leading edge flaps enabling them to be deployed and retracted without ever actuating the trailing edge flaps. However, this solution, by itself, has also been found deficient because, like the two lever design mentioned above, it puts too much reliance on the ability of the pilot to make the correct control decision under critical and perhaps emergency circumstances.
In particular, consider the case of an emergency go-around. The pilot could, under such circumstances inadvertently move the lever from full leading edge and trailing edge flap deployment, all the way back to the full retract position, thereby immediately commanding full retraction of both leading and trailing edge flaps. Responsively, the hydraulic subsystems that effect the actual positioning of the flaps would commence to simultaneously draw both leading and trailing edge flaps to their fully retracted positions. A slower reaction time of the hydraulically effected movement of the trailing edge flaps, compared to the leading edge flaps, would cause the later to become fully retracted at a time well in advance of full retraction of the former.
Because of aerodynamic considerations, an airfoil configuration on certain aircraft in which the trailing edge flaps remain deployed at a time during which the leading edge flaps are fully retracted, results, at least during emergency go-around conditions, in nonoptimum lift performance. The opportunity to cause such inadvertent movement of the flaps to this particular configuration should be, if at all possible, eliminated.
Accordingly, it is an object of the invention to provide a flap control system, operated in response to a single, pilot controlled lever, that enables a hydraulic subsystem associated with the leading edge flaps to be actuated independently of a hydraulic subsystem associated with the trailing edge flaps, so that the leading edge flaps can be separately deployed while the trailing edge flaps remain retracted; allows concurrent deployment of both leading and trailing edge flaps; and eliminates the opportunity for inadvertently commanding prior retraction of the leading edge flaps before the trailing edge flaps have been hydraulically drawn back into their retracted position.
A related object is to provide such a flap control system that can be readily, and inexpensively retrofitted to existing hydraulic subsystems that are provided for effecting the movement of the flaps in response to a pilot controlled command lever.